Optical signals are distorted as they propagate in fluctuating media like atmospheric turbulence. A system to correct the distortion, known as adaptive optics (AO), is employed to optimize system performances in the presence of the random disturbances. Such adaptive wavefront correction is usually achieved by introducing a spatially varying pattern of optical path differences across the aperture of the receiving optical system using Deformable Mirror (DM) technology.
As a critical element to such systems, deformable mirrors offer the greatest promises for high performance wavefront correction. Conventional deformable mirrors use multiple bulk piezoelectric actuators or magnetic actuators to deform large plate mirrors. While these mirrors offer high accuracy and can conform to wavefront distortion associated with broad or narrow band spectrums, they tend to be bulky, heavy, expensive, and typically operate with less than 1000 actuators at relative slow speed.
An emerging generation of deformable mirror (DM) technologies based on Micro-Electro-Mechanical Systems (MEMS) manufacturing is promising to lead to DM components with capabilities exceeding those of conventional DMs while, at the same time, reducing cost, weight, and power electronics requirements. However, MEMS-based deformable mirror technologies have been successful in small stroke applications. Large stroke deformable mirror systems using MEMS actuators have not yet demonstrated sufficient attractive combinations of high stroke, low voltage, and with high system reliability.
U.S. Pat. No. 6,384,952 to Clark et al. (2002), incorporated herein by this reference, discloses a continuous-face-sheet DM that employs a mirrored membrane fabricated, for example, from metal-coated silicon nitride and actuated by an array of vertical comb actuators disposed underneath the membrane. Use of vertical comb actuators can provide higher force and larger stroke for a given applied voltage than the parallel plate electrostatic actuators used in other continuous-face-sheet designs. However, this design requires placing vertical two teeth sets precisely relative to each other, one on the substrate and the other suspended on a membrane member, respectively, thus is unduly complicated in manufacturing the DM structure. Moreover, because of the electrostatic actuation, the device does not offer sufficient actuation force and stroke to meet the stringent requirements of a deformable mirror device for adaptive optics applications.
U.S. Pat. No. 7,336,412 to Yang (2008), also incorporated herein by this reference, describes a microcontrollable, deformable mirror comprising of a mirror membrane under which a plurality of controllable piezoelectric microactuators is coupled to the mirror membrane. Each piezoelectric microactuators comprises a pedestal, a piezoelectric microactuator, and a supporting substrate. The piezoelectric actuator structure is mounted on the supporting substrate and has an unstressed plane and has electrodes defined on opposing surfaces so that in-plane stresses electrically induced in its piezoelectric layer cause the actuator membrane to bend out of the unstressed plane in a selected direction. In the prior invention, the pedestal is connected to the mirror membrane to couple deformation of the piezoelectric actuator into substantially local deformation of the mirror membrane. However, because each of the piezoelectric actuator is mechanically a continuous membrane structure having the pedestal located at or near the membrane center, the bending deformation of the piezoelectric actuator induced by an in-plane stresses is significantly restricted.